I'd say my favorite 3 are Elegon, Vizier, and Feng, in that order. They all have a good mix of mechanics with minimal annoying RNG nor are they 15 minutes long just because (haven't done them all in heroic yet though).

Only ones I really don't like all that much are Garalon and Wind Lord.

My favorite (and I'm talking normal and haven't done them all yet) is Elegon. It's my first expansion as a raid leader, and it's me and another guy in my guild leading raids most of the time. The cool thing is that both teams kind of progressed at the same rate, so instead of copying what he was doing, I made my own tactic and raid calls on that fight. Combined with a group that steadily progressed through the fight, and really worked together, it was such a relief to see it go down and my hard working paying off in the end.

The last phase of Blade Lord is a lot of fun. I cant say I am loving many other fights in Heart of Fear. They just dont blow me away. However, I also really like Elegon and Spirit Kings and Will of the Emperor. Really fun and engaging encounters.

It's really difficult to pick a single favourite, in all honesty. This tier has been generally outstanding, easily one of the best in WoW's history. Some of the particular stand out parts though, from the perspective of a 3/6H 6/6N 4/4N Brewmaster:

MSV:

Stone Guard. The taunting mechanic, while seeming simple on the surface, is actually rather tricky to pull off successfully. Felt very satisfying to learn and pull this off.

Feng: Honestly, I can't decide if this boss is very well designed or clunky and awkward. It's got these weird qualities to it that sometimes you think 'omg that's so cool' (Shroud of Reversal, Barrier), and sometimes you just think 'what the hell, is it really meant to work that way?' (Draw Flames, Arcane Resonance). I enjoy it though, so worth mentioning.

Gara'jal: I just love this boss because it's basically designed perfectly for Brewmasters. Being able to avoid a ton of attacks, thus reduce the damage Voodoo doll'd people take significantly, then when banished easily deal with your Severer of Souls with a swift Touch of Death... It's just perfect for us.

Elegon: Just, Elegon. <3

Will of the Emperor: Dancing! Brilliant mechanic.

Heart of Fear:

Wind Lord Mel'jarak: being able to top both damage done and healing done, AS A TANK, is roflworthy. Again, a fight perfectly designed for Brewmasters.

Amber-shaper: A very fun and 'different' fight, though I keep getting flashbacks of Teron Gorefiend...

Shek'zeer: Phases 1 and 3 are a bit dull, but phase 2 with the adds? Pretty pulse-pounding as a tank. Doing everything you can to survive, pushing yourself to your limits and co-ordinating with your raid... Very fun.

Terrace:

Sha of Fear: I love being able to hax Thrash as a Brewmaster, with Elusive Brew. The fight in general is quite fun, but goes on for a bit too long I think. Would like it more if it was shorter.

The fights not mentioned are by no means bad; I can honestly say that there's no truly bad fights for me this tier, when usually I can think of at least 1 per tier.

probably feng and will, overall MSV is pretty nice, like the shroud and the whole dancing thing
not really a fan of HoF, kinda shitty as far as tank mechanics go, mostly just taunt at X with garalon being absolute shit, well windlord is nice to tank because of "dem numbers" :P
Terrace is fun too, but nothing too outstanding

Heart of Fear was pretty dissapointing in all honesty. It's a nice tier, not as good as Tier 11, but it's still good. Some of the bosses are just not very memorable, like Stone Guard, Mel'jarak, Tay'ak and Gara'jal. Still, they're okay encounters. The only literally bad encounter is Lei Shi, it's just a fight that tries to take as long as possible to hit enrage. Hide is stupid, focusing down one elemental is awkward because the other three are just there to be there, and Stay Away is just useless. Worst of all is her voice though, reminds me of those stupid anime chicks who are trying too hard to be cute but are fucking annoying instead.